Transparent polyurethane compositions

ABSTRACT

The present invention is embodied in a polyurethane composition suitable for coating, casting or laminating, which incorporates a prescribed additive selected to increase the composition&#39;s electrical conductivity without adversely affecting the composition&#39;s transparency and percent haze and without adversely affecting its adhesion to an underlying substrate or its environmental durability, especially with regard to humidity resistance. The prescribed additive is an ionizable metal salt of a perfluoroalkylsulfonimide, in a weight percent of 0.5 to 5.0, with the metal being an alkali metal and the preferred perfluoroalkylsulfonimide being trifluoromethanesulfonimide. The most preferred salt is lithium trifluoromethanesulfonimide. The preferred polyurethanes include both aliphatic polyetherurethanes and aliphatic polyesterurethanes. Use of the additive enhances the composition&#39;s electrical conductivity by at least about two orders of magnitude, thereby minimizing the risk of a static charge building to a point where a shock hazard is created or the polyurethane coating or laminate is damaged by a rapid discharge of electrical current. The composition is particularly useful in an aircraft window or transparency, where it can be used as a coating or laminated film overlaying a transparent conductive coating.

This is a division of application Ser. No. 09/016,452, filed Jan. 30,1998, now U.S. Pat No. 6,093,451.

BACKGROUND OF THE INVENTION

This invention relates generally to transparent polyurethanecompositions, and to coated transparencies and laminates incorporatingsuch compositions, and more particularly to such compositions, coatedtransparencies, and laminates having antistatic or static dissipativeproperties.

Polyurethanes and other organic polymers generally are poor conductorsof electricity. Consequently, these polymers cannot be usedsatisfactorily without modification in applications where staticdissipative properties are required.

Several methods have been used in the past to modify polyurethanes so asto increase their electrical conductivity, and thereby to betterdissipate a buildup of static charge. In one such method, conductivefibers or particles are incorporated into the polyurethane matrix. Thismethod is not suitable for use with polyurethanes that are transparent,however, because the conductive filler materials render the modifiedpolyurethane opaque.

In another method for modifying polyurethanes to increase theirelectrical conductivity, conductive polymers based on polyanilines areincorporated into the polyurethane matrix. Again, however, this methodis not suitable for use with polyurethanes that are transparent, becausethe polyaniline additives form a dispersed phase that reduces thepolyurethane's transparency. In addition, polyanilines generally areineffective at increasing the modified polyurethane's conductivity whenincorporated at a low concentration.

In yet another method for modifying polyurethanes to increase theirelectrical conductivity, hydrophilic additives such as amines andquaternary ammonium salts are used to increase the polyurethane'ssurface conductivity. These additives function by migrating to thepolyurethane's surface, where they attract water and thereby create aconductive film. This method is not suitable for polyurethane coatingsand laminates, however, because the additive also migrates to thesurface of the polyurethane that interfaces with the underlyingsubstrate, to cause a loss of adhesion. In addition, such additives canlose their effectiveness over time, because they can leach from thepolyurethane under normal use conditions.

Still other methods for modifying polyurethanes to increase theirelectrical conductivity, usable in the past only for polyurethane foams,call for adding in ionizable metal salts coupled with an enhancer. Thepreferred salt cation is an alkali or alkaline earth metal ion, and thepreferred anion is the conjugate base of an inorganic acid or a C2-C4carboxylic acid. The preferred enhancers are phosphate esters and saltsor esters of fatty acids.

None of these known additives for increasing the electrical conductivityof polyurethanes are considered fully satisfactory for use inpolyurethanes that are transparent, and particularly in polyurethanesthat are used as coatings or in laminates for aircraft windows.

In general, non-ionic additives and polyol modifiers have been found tosignificantly enhance electrical conductivity only if used at highlevels, which can adversely affect other important properties, such astransparency and mechanical strength. Ionic additives, includingquarternary ammonium salts and ionizable metal salts, generally are moreeffective in enhancing electrical conductivity. The most effective knownadditives of this kind are ionizable metal salts ofperfluoroalkylsulfonates. However, none of these ionic additives areconsidered fully satisfactory for use in transparent polyurethanes usedas coatings or in laminates, because with aging they can cause a loss oftransparency and a loss of adhesion.

It should therefore be appreciated that there is a need for an improvedpolyurethane composition, and for coated transparencies and laminatesincorporating such a composition, that incorporates an additive forenhancing electrical conductivity without adversely affecting thecomposition's transparency and without adversely affecting thecomposition's adhesion to an underlying substrate. The present inventionfulfills this need and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention is embodied in an improved polyurethanecomposition, and in coated transparencies and laminates (e.g., aircraftwindows) incorporating such a composition, the composition incorporatinga prescribed additive for enhancing electrical conductivity withoutadversely affecting the composition's transparency and or adhesion to anunderlying substrate. More particularly, the polyurethane compositionincorporates 0.5 to 5.0 weight percent of an ionizable salt of aperfluoroalkylsulfonimide. The metal preferably is an alkali metal, andthe perfluoroalkylsulfonimide preferably is trifluoromethanesulfonimide.The most preferred ionizable salt is lithiumtrifluoromethanesulfonimide, in a weight percent in the range of 1.0 to3.0. This enhances the composition's electrical conductivity by at leastabout two orders of magnitude.

The ionizable salt of a perfluoroalkylsulfonimide is suitable for use asan additive in both polyesterurethanes and polyetherurethanes. Thepreferred polyurethane composition incorporates aliphaticpolyetherurethane, with which the prescribed additive can reduceelectrical volume resistivity to values of less than about 1×10¹¹ohm-cm.

One important use of the polyurethane composition of the invention is asa transparent coating or as part of a transparent laminate. When used asa transparent coating, the composition has particular utility whencoated onto a transparent, conductive metal coating, e.g., indium tinoxide (ITO), gold, and gold/metal oxide stacks, overlaying a transparentsubstrate. The enhanced electrical conductivity minimizes the risk thatstatic charge can build to a point where a shock hazard is created orthe polyurethane coating is damaged by a rapid discharge of electricalcurrent.

Other features and advantages of the invention should become apparentfrom the following description of the preferred embodiments, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the relationship between volume resistivityand temperature, for the polyurethane compositions of Examples 15 and21.

FIG. 2 is a graph depicting the relationship between volume resistivityand temperature, for the polyetherurethane compositions of Examples 20and 21.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is embodied in a polyurethane composition thatincorporates a prescribed additive selected to increase thecomposition's electrical conductivity (i.e., decrease its volumeresistivity) without adversely affecting the composition's transparencyand environmental durability, or the transparency's adhesion to anunderlying substrate. The prescribed additive is an ionizable metal saltof a perfluoroalkylsulfonimide, with the metal being an alkali metal andthe preferred perfluoroalkylsulfonimide beingtrifluoromethanesulfonimide. The most preferred salt is lithiumtrifluoromethanesulfonimide. The preferred polyurethanes include bothaliphatic polyetherurethanes and aliphatic polyesterurethanes.

The modified polyurethane composition is ideal for use as a coating fora transparent substrate or as part of a transparent laminate. Thecomposition's enhanced conductivity minimizes the possibility of thebuildup of static charge where a shock hazard is created or damage tothe polyurethane can occur.

The modified polyurethane composition of the invention can best beunderstood by reference to the following examples.

EXAMPLES 1-15 Preparation of Transparent PolyesterurethanesIncorporating Antistatic Additives

A transparent aliphatic polyesterurethane mix, suitable for coating,laminating or casting, was prepared from the formulation set forth inTable 1.

TABLE 1 Aliphatic Polyesterurethane Formulation Raw Material DescriptionParts (wt.) Desmodur W¹ Bis(4-isocyanato- 38.62 cyclohexyl)methane Tone305² Polycaprolactone triol 43.88 Tone 210² Polycaprolactone diol 17.20Butanediol Chain extender 0.36 Tinuvin 328³ UV stabilizer 0.50 Irganox1010³ Antioxidant 0.50 ¹Available from Bayer ²Available from UnionCarbide ³Available from Ciba Geigy

After a homogeneous solution of the specified polyesterurethane mix wasprepared, with the solution temperature at 100° F., 30 ppm dibutyl tindilaurate catalyst and an antistatic additive were added. Fifteen suchsolutions were prepared, using the antistatic additives identified inTable 2. The resulting solutions were then cast onto glass or anothersuitable surface with a transparent conductive coating and cured at 120°F. for 12 hours, followed by 180° F. for 24 hours. The cast films wereabout 0.025 inches thick. After curing, the volume resistivity of eachof the 15 formulations was measured (at 60° F.) using the methoddescribed in ASTM D-257. The additives, their weight percentage, and theresulting electrical resistivity measurements are presented in Table 2.

TABLE 2 Antistatic Additives in Polyesterurethane Vol. Res. Ex. AdditiveType Wt. % (ohm-cm) 1 none — 0 >10¹⁴ 2 Cyagard SN¹ Ammonium salt 1.0 1.3× 10¹³ 3 Markstat AL 12² Ammonium salt 1.0 5.0 × 10¹³ 4 Markstat AL 12²Ammonium salt 4.0 1.3 × 10¹² 5 Antistaticum RC100³ Polyether polyol 1.0>10¹⁴ 6 Antistaticum RC100³ Polyether polyol 10.0 4.8 × 10¹² 7 StatureII⁴ Polyether polyol 1.0 >10¹⁴ 8 Stature II⁴ Polyether polyol 10.0 9.2 ×10¹² 9 Larostat 377⁵ Ammonium salt 1.0 1.5 × 10¹³ 10 KenStat KSMZ100⁶Zirconate salt 1.0 1.9 × 10¹³ 11 Atmer 154⁷ Fatty acid ester 1.0 >10¹⁴12 Atmer 154⁷ Fatty acid ester 10.0 1.5 × 10¹³ 13 Versicon⁸ Polyaniline1.0 >10¹⁴ 14 Fluorad FC122⁹ Lithium trifluoro- 1.0 4.5 × 10¹² methanesulfonate 15 Fluorad HQ115⁹ Lithium trifluoro- 1.0 3.1 × 10¹²methanesulfonimide ¹Available from Cytec ²Available from Witco³Available from Bayer ⁴Available from Dow ⁵Available from PPG ⁶Availablefrom Kenrich ⁷Available from ICI ⁸Available from Allied Signal⁹Available from 3M

Examples 1-15 show that lithium trifluoromethanesulfonimide is the mosteffective of the identified antistatic additives in reducing volumeresistivity of a transparent polyesterurethane of the kind suitable foruse as a coating or as part of a laminate. Volume resistivity is reducedby substantially more than it is reduced by other additives, and bynearly two orders of magnitude over what it would have been without anyadditive.

The volume resistivities of the polyesterurethane compositions ofExamples 1-15 all vary inversely with temperature, thus increasing withdecreasing temperature. FIG. 1 is a graph showing the specificdependence of volume resistivity on temperature for thepolyesterurethane composition of Example 15.

EXAMPLES 16-21 Preparation of Transparent PolyetherurethanesIncorporating Antistatic Additives

A transparent aliphatic polyetherurethane mix, suitable for coating orcasting, was prepared from the formulation set forth in Table 3.

TABLE 3 Aliphatic Polyetherurethane Formulation Raw Material DescriptionParts (wt.) Desmodur W¹ Bis(4-isocyanato- 38.06 cyclohexyl)methaneTerathane 1000² Polytetramethylene oxide diol 54.48 Trimethylol propaneTriol 7.46 Tinuvin 328³ UV stabilizer 0.50 Cyagard 1164⁴ UV stabilizer0.50 Irganox 1010³ Heat stabilizer 0.50 Sanduvor 3055⁵ Light stabilizer0.50 ¹Available from Bayer ²Available from DuPont ³Available from CibaGeigy ⁴Available from Cytec ⁵Available from Clariant

After a homogeneous solution of the specified polyetherurethane mix wasprepared, with the solution temperature at 100° F., 30 ppm dibutyl tindilaurate catalyst and an antistatic additive were added. Six suchsolutions were prepared, using the antistatic additives identified inTable 4. The resulting solutions were then cast onto glass or anothersuitable surface with a transparent conductive coating and cured at 120°F. for 12 hours, followed by 180° F. for 24 hours. The cast films wereabout 0.025 inches thick. After curing, the volume resistivity of eachof the six formulations was measured (at 60° F.) using the methoddescribed in ASTM D-257. The additives, their weight percentage, and theresulting resistivity measurements are presented in Table 4.

TABLE 4 Antistatic Additives in Polyetherurethane Vol. Res. Ex. AdditiveType Wt. % (ohm-cm) 16 none — 0 1.3 × 10¹³ 17 Cyagard SN¹ Ammonium salt1.0 1.0 × 10¹² 18 KenStat KSMZ100² Zirconate salt Not soluble inpolyurethane 19 Versicon³ Polyaniline 1.0 1.3 × 10¹³ 20 Fluorad FC122⁴Lithium trifluoro- 1.0 1.9 × 10¹¹ methane sulfonate 21 Fluorad HQ115⁴Lithium trifluoro- 1.0 1.7 × 10¹⁰ methanesulfonimide ¹Available fromCytec ²Available from Kenrich ³Available from Allied Signal ⁴Availablefrom 3M

Examples 16-21 show that lithium trifluoromethanesulfonimide not only isthe most effective of the identified antistatic additives in reducingvolume resistivity of a transparent polyesterurethane, but also of atransparent polyetherurethane. Volume resistivity is reduced bysubstantially more than it is reduced by other additives, and by wellmore than two orders of magnitude over what it would have been withoutany additive.

FIG. 2 is a graph showing the dependence of volume resistivity ontemperature for the polyetherurethane compositions of Examples 20 and21. Also included in FIG. 2 are curves showing the dependence of volumeresistivity on temperature for polyetherurethane compositions containing3% lithium trifluoromethanesulfonate (Fluorad FC122), and 2% and 3%lithium trifluoromethanesulfonimide (Fluorad HQ115). It will be notedthat increasing the concentration of lithium trifluoromethanesulfonimideto 2% causes a marked reduction in volume resistivity, but thatincreasing the concentration further to 3% has little, if any,additional effect. The resistivity/temperature curve for the compositionof Example 21 is repeated in FIG. 1, so that the electrical resistivityof polyesterurethane and polyetherurethane, both containing lithiumtrifluoromethanesulfonimide (Fluorad HQ115), can be readily compared.

EXAMPLES 22-27 Durability Testing of Modified Polyurethanes

It is noted from Tables 2 and 4 that the particular additives thatyielded the largest increase in conductivity in the two types ofpolyurethanes were Fluorad FC122 (lithium trifluoromethanesulfonate) andFluorad HQ115 (lithium trifluoromethanesulfonimide). In Examples 22-27,mixes of both polyesterurethane and polyetherurethane containing theseadditives, in weight percentages of both 1% and 3%, were prepared asspecified above and then coated onto polycarbonate sheet samples thathad been pre-coated with a transparent, conductive coating and primer.

The pre-coated polycarbonate had been prepared using aircraft-gradepolycarbonate sheet (MIL-P-83310) with a conventional polysiloxane hardcoating. These sheets were first coated with a three-layer indium tinoxide (ITO)/gold/ITO film stack, using a sputtering process, to a sheetresistance of about 15 ohms/square. A silica (SiO₂) layer about 30nanometers in thickness was then deposited over the ITO coating using anevaporation process. Finally, an aminosilane primer was then applied byflow coating using a 0.1% solution of A-1100 primer (Union Carbide)dissolved in isopropanol.

After curing, the coated samples were exposed to 95%-100% relativehumidity, at a temperature of 120° F., and light transmission, haze, andadhesion were measured at 500-hour intervals. The samples incorporating3% additive concentration levels were included to represent worst-caseconditions, and to accelerate the rate of processes causing propertydegradation. The results of these tests are presented in Table 5.

TABLE 5 Humidity Testing of Polyurethane Coatings Exp Time, Additive,p-urethane¹ hrs² % LT³ % Haze³ Adhesion⁴ 1% FC122, p-ester 0 82.5 1.5100% 500 75.3 >50 Delaminated 1% HQ115,p-ester 0 83.1 1.4 100% 500 83.01.6 100% 2000 83.0 1.6 100% 1% FC122, p-ether 0 83.1 1.3 100% 50077.2 >50 Delaminated 1% HQ115, p-ether 0 79.7 1.2 100% 500 79.3 1.6 100%2000 78.7 1.8 100% 3% FC122, p-ether 0 83.3 1.0 100% 500 Opaque OpaqueDelaminated 3% HQ115, p-ether 0 81.8 1.5 100% 500 81.9 1.8 100% 200081.9 1.6 100%

Percent light transmission and percent haze were measured according toASTM D-1003, and percent adhesion was measured according to ASTM D-3359.

Examples 22-27 show the surprisingly better humidity resistance oftransparent polyurethane coatings modified to incorporate lithiumtrifluoromethanesulfonimide, as compared with the same polyurethanesmodified with lithium trifluoromethanesulfonate. In particular, thecoatings incorporating lithium trifluoromethanesulfonimide exhibitedsubstantially no degradation in light transmission, percent haze andpercent adhesion during the humidity tests. In contrast, the coatingsincorporating lithium trifluoromethanesulfonate exhibited substantialdegradation in light transmission and percent haze after just 500 hoursof the humidity tests, and the coatings all delaminated from theirunderlying substrates.

EXAMPLES 28-30 Transparent Antistatic Polyetherurethane Bilayer Coatingson ITO-Coated Glass, Polycarbonate, and Acrylic

In Example 28, a soda lime glass surface was coated with ITO, silica,and an aminosilane primer, as described above in Examples 22-27. To forma first polyurethane coating for this glass surface, an adhesivesolution was prepared from a moisture-curable, aromaticpolyetherurethane, thermoset adhesive, based on methylene diphenyldiisocyanate (MDI) and polytetramethylene oxide polyols, incyclohexanone at 18% concentration. Lithium trifluoromethanesulfonimide(Fluorad HQ115) was then dissolved in the adhesive solution at aconcentration of 1% based on the polyurethane, and this adhesivesolution was then flow coated onto the ITO/SiO₂-coated glass surface andcured at room temperature for 3-4 hours.

To form a second polyurethane coating over the first polyurethanecoating, a polyesterurethane solution was prepared as set forth in Table1, and 1% lithium trifluoromethanesulfonimide (Fluorad HQ115) and 200ppm dibutyl tin dilaurate catalyst were then added. The resultingsolution was then coated over the first polyurethane coating and curedat 100-140° F. for two hours followed by 180° F. for 24 hours. Aftercuring, the resulting bilayer coating system was determined by themethod of ASTM D-257 to have a volume resistivity of 3.2×10¹² ohm-cm.

In Example 29, ITO and SiO₂ coatings were applied to the surface of apolysiloxane-coated polycarbonate sheet using the procedure describedabove in Examples 22-27. A bilayer polyurethane coating using 1% FluoradHQ115 was then applied to the surface and cured as described in Example28. The volume resistivity of this bilayer coating system was determinedby the method of ASTM D-257 to be 3.1×10¹² ohm-cm.

In Example 30, a three-layer ITO/gold/ITO stack and a SiO₂ coating wereapplied to a polysiloxane-coated acrylic sheet, using the proceduredescribed in Examples 22-27. The acrylic sheet was aircraft grade perMIL-P-25690. A bilayer polyurethane coating using 1% Fluorad HQ115 wasthen applied to the surface and cured as described in Example 28. Thevolume resistivity of this bilayer coating system was determined by themethod of ASTM D-257 to be 3.5×10¹² ohm-cm.

Examples 28-30 show the effectiveness of a bilayer polyurethane coatingsystem using a thermoset adhesive and applied over various transparentsubstrates.

EXAMPLES 31-32 Transparent Antistatic Polyurethane Bilayer Laminate

In Example 31, a first polyurethane sheet was prepared by forming asolution at 100° F. based on the formulation set forth in Table 6. Addedto this solution was 1% Fluorad HQ115, followed by 200 ppm dibutyl tindilaurate catalyst. The mix was then pumped into a cell formed from twoglass sheets and a peripheral gasket, and cured at 180° F. for 24 hours.After separation from the glass, a transparent polyurethane sheet ofabout 0.040 inches thickness was obtained. The volume resistivity ofthis cast thermoset sheet was determined by the method of ASTM D-257 tobe 4.9×10¹² ohm-cm.

TABLE 6 Aliphatic Polyesterurethane Formulation Raw Material DescriptionParts (wt.) Desmodur W¹ Bis(4-isocyanato- 47.0 cyclohexyl)methane Tone301² Polycaprolactone triol 22.0 Tone 200² Polycaprolactone diol 30.0Tinuvin 328³ UV stabilizer 0.5 Irganox 1010³ Antioxidant 0.5 ¹Availablefrom Bayer ²Available from Union Carbide ³Available from Ciba Geigy

A second polyurethane sheet was then prepared by dissolving FluoradHQ115 in isopropanol, at a concentration of 1%, and this solution wasthen sprayed evenly onto both surfaces of a 0.025-inch thick extrudedtransparent aliphatic polyetherurethane sheet (PE-399 manufactured byMorton). Sufficient solution was applied to yield a total concentrationof 1% Fluorad HQ115 based on the polyurethane sheet. The sheet was airdried for 16 hours, to evaporate all of the solvent The volumeresistivity of this modified sheet was determined by the method of ASTMD-257 to be 1.5×10¹⁰ ohm-cm.

The first and second polyurethane sheets described above were thenlaminated to a polycarbonate sheet having a conductive ITO/gold/ITOstack and SiO₂ coatings, as described above in Examples 22-27.Specifically, the second polyurethane sheet, i.e., the thermoplasticaliphatic polyetherurethane containing 1% Fluorad HQ115, was used as anadhesive between the polycarbonate sheet and the first polyurethanesheet, i.e., the polyesterurethane sheet. The three sheets werelaminated in an autoclave using conventional vacuum bag techniques, at atemperature of 200° F. and pressure of 100 psi. The volume resistivityof this bilayer laminate was determined by the method of ASTM D-257 tobe 1.2×10¹² ohm-cm.

In Example 32, a laminate was prepared as described above in Example 31,except that the first polyurethane sheet was formed of polyetherurethane(PE-399, Morton), with Fluorad HQ115 incorporated into the resin priorto extrusion at a 1% level. The volume resistivity of the resultingextruded sheet was determined by the method of ASTM D-257 to be 1.8×10⁹ohm-cm. The second polyurethane sheet and the coated polycarbonate sheetwere the same as in Example 32, and the various layers were laminatedtogether in the same manner as was done in Example 32. The volumeresistivity of this bilayer laminate was determined by the method ofASTM D-257 to be 1.5×10¹² ohm-cm.

Examples 31 and 32 show the effectiveness of the polyurethanes of theinvention used as bilayer laminates with a thermoplastic adhesive. Theexamples also show alternate methods of incorporating the preferredantistatic additive into transparent polyurethanes.

It should be appreciated from the foregoing disclosure that the presentinvention provides an improved polyurethane composition thatincorporates a prescribed additive for enhancing electrical conductivitywithout adversely affecting the composition's transparency and oradhesion to an underlying substrate. The additive enhances thecomposition's electrical conductivity by at least about two orders ofmagnitude, making it ideally suited for use as a coating for atransparency, or as part of a laminate, that can avoid shock hazards ordamage from a buildup of static charge.

Although the invention has been disclosed with reference only to thepresently preferred embodiments, those of ordinary skill in the art willappreciate that various modifications can be made without departing fromthe invention. Accordingly, the invention is defined only by thefollowing claims.

We claim:
 1. A transparent polyurethane composition suitable for coatingor casting, wherein the composition consists essentially of polyurethaneand an ionizable salt of a perfluoroalkysulfonimide, and wherein theionizable salt of the perfluoroalkysulfonimide is present in an amount0.5 to 5.0 weight percent of the total amount of polyurethane andionizable salt of the perfluoroalkysulfonimide in the composition.
 2. Atransparent polyurethane composition as defined in claim 1, wherein thecomposition is an aliphatic polyesterurethane.
 3. A transparentpolyurethane composition as defined in claim 1, wherein the compositionis an aliphatic polyetherurethane.
 4. A transparent polyurethanecomposition as defined in claim 1, wherein the metal is an alkali metaland the perfluoroalkylsulfonimide is trifluoromethanesulfonimide.
 5. Atransparent polyurethane composition as defined in claim 4, wherein theionizable salt is lithium trifluoromethanesulfonimide.
 6. A transparentpolyurethane composition as defined in claim 5, wherein the weightpercent of lithium trifluoromethanesulfonimide is in the range of 1.0 to3.0.
 7. A transparent polyurethane composition as defined in claim 1,wherein the ionizable salt is incorporated at a concentration effectiveto reduce the composition's volume resistivity, after curing, by morethan about two orders of magnitude over what it would be in the absenceof the ionizable salt.